Generally, income from long term disability insurance is not taxable because the money you used to pay your premiums has already been taxed. This increases the value of your long term disability payments substantially, to not have taxes taken out, and is a great feature of long term disability insurance.
If you pay premiums for your own disability policy then the benefit will not be taxable.
However, if you have a group long term disability policy that is paid for by your employer then those premiums are taxed as ordinary income.